There is known a holder for aligning and holding plural single-core optical fibers. Specifically, there is known a holder for inserting four single-core optical fiber core wires from a gap between a lower surface of a first clamp and a flat bottom surface of a first groove part formed in a first base part into the first groove part, thereby holding the four optical fiber core wires at the first groove part such that they mutually contact and they are parallel (for example, see Patent Reference 1).